A ""medical hostage"" is precisely what Secretary of Defense Kramer becomes once the vice-presidential candidate is injected with an unknown substance while making a campaign rally in his hometown Chicago. Kramer's condition deteriorates steadily as the toxicologists at Presbyterian Hospital scurry about trying to identify a chemical agent which could be anthrax or botulism or snake venom--but isn't. What it is is Z-3, a new viral nerve toxin, and its antidote is bartered for the freedom of a half dozen convicts serving life sentences. The M.D. author knows his stuff but the work-up isn't all that exciting.